mitosis

Mitosis is the act of a single character splitting in two or more characters.

Variants

Biological Mitosis - The body of the character is depicted physically changing through transformation. This often results in a conjoined mid_transformation scene.

Goo Mitosis - The body melts, splits into pieces, and reconstitutes into two or more entities.
Gory Mitosis - The body, as it currently is, rips itself apart. The pieces then gradually regenerates into their own individuals.
Metamorphic Mitosis - The character transforms into a pupa or wraps itself inside a cocoon in which two or more creatures emerge.

Supernatural Mitosis - Through some esoteric means, such as magic, the character achieves mitosis.

Energy Mitosis - The body is converted into light or pure energy, splits into pieces, and rematerializes into two or more entities.
Instant Mitosis - The character is suddenly now two or more people.

Tagging Familial Relations

As with the nature of this form of reproduction, the parent is essentially destroyed in the process and their biomass distributed among the resultant offsprings to create them so a parent_(lore) tag won't be necessary, unless they're present in the post as part of a sequence.

As for the new entities; Due the fact that they originated from the same character, they would essentially be twin/triplet/etc. siblings so a sibling_(lore) and a twins_(lore) tag would be necessary. The sex, among many other things, is often inherited from the original so it'll be either brothers_(lore) or sisters_(lore). Sometimes the split results in distinct offsprings so a brother_and_sister_(lore) can happen.

What follows this would be familial relations similar to how a new generation is related to the original's relatives such as the original's parents are their grandparents_(lore), their brother/sister is their uncle_(lore)/aunt_(lore), etc.

Not to be confused with

Budding - A smaller chunk breaks off from the parent's body which then grows and develops into a new entity; Their offspring. The parent remains relatively alive and intact albeit with reduced biomass.
Merging -Two or more characters merging together into one being.